The present invention relates to a dishwasher filtration and soil collection system, and more particularly to a system for automatically purging a filter and soil collection system in a dishwasher to remove accumulated soils.
Typical domestic dishwashers in use today draw wash liquid from a sump at the bottom of a wash tub and spray the wash liquid within the wash tub to remove soils from dishes located on racks in the tub. In an attempt to improve performance and efficiency, some dishwashers employ a system for separating soil out of the recirculating wash liquid and for retaining the soils in a collection chamber. Frequently, a filter screen is used to retain soil in a soil collection chamber. U.S. Pat. No. 5,165,433, for example, discloses a dishwasher system including a centrifugal soil separator which sends soil laden wash liquid into a soil container whereupon the soil laden wash liquid passes through a fine filter disposed in the wall of the soil container.
Inherent in the system described in the '433 patent, and in any fine mesh filter screen system in a dishwasher, is the problem of screen clogging by food soils removed from the dishes. Typically, backwash jets are directed against the filter in an attempt to clear the filter and prevent clogging. Heavy soil loads, however, can result in screen clogging in spite of backwash jets.
Screen clogging can adversely affect the dishwasher's cleaning ability, causing poor washability and indirectly causing increased water and energy consumption. Moreover, the build-up of pressure behind the screen may increase--to a maximum determined by the ability of the pump supplying soil laden wash liquid against the screen--and result in soil embedding into the screen such that it is difficult to subsequently remove the soils from the screen.
Some attempts have been made to develop a dishwasher wash system which is capable of dealing with heavy soil loads and avoid filter clogging. U.S. Pat. No. 4,559,959 discloses a dishwasher wherein soil load is measured by monitoring pressure in a soil collection chamber in which soils are retained after the wash liquid passes through a filter mesh. If the pressure exceeds a predetermined limit, indicating that the filter mesh is clogged, the wash liquid is completely purged by draining all of the wash liquid out of the tub and refilling the tub with fresh water. The '959 patent provides for a maximum of three complete purges at the beginning of the dishwasher cycle. Additionally, the number of purges required is monitored and that information is used to control the subsequent wash cycle--selecting the appropriate cycle for the soil load of the dishes.
Concerns over dishwasher water and energy consumption make complete purges of wash liquid from a tub undesirable. Accordingly, some dishwasher systems utilize purges which only partially drain the dishwasher tub. For example, U.S. Pat. No. 4,346,723 discloses a dishwashing system wherein soils are collected in a bypass soil collector. The soil collector may be purged by draining small amounts of wash liquid in "spurts" during an early wash period by selectively opening and closing a drain valve.
U.S. Pat. No. 5,223,042 discloses a method of washing dishes wherein during the wash cycle a portion of the washing solution is drained from the bottom of the tub to remove soils. The wash solution is subsequently replenished with fresh water having a volume equal to the volume of the discharged wash solution.
U.S. Pat. No. 5,429,679 includes a soil collection system wherein wash liquid is sent into a filtration chamber and then returned to the tub sump through a filter. After the first wash cycle, a portion of wash liquid, approximately 1 gallon out of the total 2.3 gallons of wash liquid, is sent to drain and then replaced by adding fresh water to the tub.
The above described systems all include several drawbacks. One of the most significant is that, for all of these references, a relatively large quantity of water is drained during each purge. Moreover, several of the above references teach interrupting the wash operation during each drain purge such that no spray is directed against the dishes while wash liquid is being purged. Another problem with the above described systems is one of soil redeposition wherein soils, collected in the soil collection chamber prior to each purge, are redeposited onto the dishes during the purge cycle.
In addition to the inadequacies of the prior art in dealing with clogging filter screens, there exists a need for a dishwasher having improved energy efficiency. As discussed above, the need for a dishwasher which high efficient in its use of water and power is well understood. One of the functions of a dishwasher is to provide mechanical energy for soil removal by pumping water through a spray system for application against soiled dishes. An efficient dishwasher, therefore, requires a highly efficient pump.
It is well known that volute type pumps, wherein a centrifugal pump is housed in a spiral casing so that rotational speed will be converted to pressure without shock, are highly efficient pump designs. This type of pump is used extensively in dishwashers because of its efficiency, see for example U.S. Pat. No. 4,243,431 and U.S. Pat. No. 5,268,334. Another type of pump extensively used in dishwashers are vertical axis pump systems where the flow of wash liquid is perpendicular to the plane in which the pump impeller rotates, such as the pump system disclosed in the '433 patent. These types of vertical axis pumps where flow is normal to the rotation of the impeller are less efficient than volute type pumps in a dishwasher. However, the soil separation systems, discussed above, that have been developed for use with vertical axis pump systems in dishwasher make these vertical axis pump systems operate in a highly efficient and effective manner. For example, the soil separation system disclosed and claimed in U.S. Pat. No. 5,803,100, to Thies, provides for a very efficient separation of soils from the recirculating wash liquid in a dishwasher such that the overall dishwasher efficiency is increased.
It can be understood therefore, by one skilled in the art, that there is a need for a dishwasher which is capable of recirculating wash liquid through the dishwasher, removing soils from dishware and sending the removed soils to drain in an effective and highly efficient manner.